Apnea is common in premature infants, and is sometimes called apnea of prematurity. It is related to immaturity of the central nervous system, although it can also occur secondary to other causes and is a common manifestation of many neonatal diseases. It occurs in more than half of infants whose birth weight is less than 1500 g, and in almost all infants whose birth weight is less than 1000 g (see N. N. Finer et al., Pediatrics (2006) 117:S47-S51). Apnea may be a cause or an effect of many other clinical problems, such as abnormal neurological development.
Three types of apnea are common in premature infants: obstructive apnea, central apnea and mixed apnea. Obstructive apnea is a blockage of the airway, typically accompanied by struggling or thrashing movements of the infant. Central apnea is cessation of respiratory drive: the infant simply stops breathing for some period of time, and usually remains very still. Mixed apneas typically begin with an obstructive event, and then change to central apnea. In many cases, the apnea is combined with, or induces, bradycardia (a significant slowing of the heart rate). The same three types of apnea occur in adults.
All of these apneas are serious clinical events that need immediate medical attention. However, existing monitors for apnea are unsatisfactory—they miss many serious events. Typical respiratory monitors do not distinguish the heart signal in chest impedance from the respiratory signal. For this reason, they often fail to recognize apnea and therefore fail to provide a warning signal to neonatal intensive care unit (“NICU”) personnel alerting them to the fact that the infant is not breathing.
Thus, there is a need in the art for improved systems and methods to provide earlier detection of apnea. The methods described herein appropriately filter out electrical fluctuations resulting from the heartbeat such that chest impedance measurements more accurately track the respiratory rate. Due to the increased detection of apnea events, in certain embodiments, automated interaction is utilized to stimulate the premature infant during an apnea event.
One way to remove the heartbeat from the chest impedance was proposed by Neil H. K. Judell in U.S. Pat. No. 4,379,460 (1983). That method assumed that the fluctuation in the chest impedance caused by the latest heartbeat is the same as that caused by the previous heartbeat. Other methods are described in U.S. Pat. Nos. 4,781,201 and 5,503,160. None of these references teaches a method of determining the probability of apnea.